SwRI Rolls out Hydrogen Demonstration Truck

October 30, 2024

Matt Wolfe

For decades, the running joke around hydrogen being a viable fuel for commercial trucks has been that its “ten years away from being ten years away.” Though H2-fueled rigs operating at scale has long seemed like a pipe dream, shifting winds around the globe blowing towards decarbonization have finally pushed this technology to be ready for the road.

With the demand for the development of new propulsion technologies rising, organizations such as the Southwest Research Institute (SwRI) have ramped up R&D efforts to make this tech commercially viable. SwRI is an independent provider of research services and can rapidly assemble teams to tackle problems. SwRI’s main mission is to push the boundaries of science and technology to develop innovative solutions.

One such project has been the development of a hydrogen-fueled Class 8 demonstration vehicle, which was on display at the 2024 SAE COMVEC conference in September. Built as part of its industry-supported H2-ICE consortium, the goal of the demonstration was the development of a hydrogen-fueled combustion engine that produced ultra-low NOx and CO2 emissions while still providing enough torque and power for most heavy-duty applications.

Avengers assemble

SwRI’s H2-ICE consortium was launched in 2022 and included engine and truck manufacturers, fuels and lubricants providers and Tier 1 suppliers with a unified vision of advancing sustainable mobility through innovative hydrogen engine technology. The consortium focused on demonstrating the potential for H2-ICE vehicles to complement other zero-emission vehicle technologies on the industry’s decarbonization roadmap.

To achieve the consortium’s goals, the engine needed to demonstrate industry-leading NOx emissions in addition to the low CO2 emissions expected from hydrogen combustion. The SwRI team set a target of meeting the California Air Resource Board’s (CARB) Ultra-Low NOx designation of 0.02 g/hp-hr.

“We wanted the program to align with the EPA’s Phase-3 greenhouse gas policy, so we knew our timeline was ambitious,” said Ryan Williams, SwRI Powertrain Engineering Division manager and the H2-ICE consortium’s program manager. “It took incredible planning by the integration teams to ensure that the build proceeded smoothly.”

SwRI converted an X15N natural gas engine provided by Cummins to run on port-injected hydrogen. Allison provided a transmission for the truck, while Forvia provided the H2 tanks. Other OEMs and Tier 1s that supplied components and knowledge include Bosch, Phinia, SuperTurbo, Mahle, SEM, Woodward and Eaton.

“From custom-built parts and prototype components to specially formulated lubricants, this has truly been an industry-wide effort,” Williams said. “We could never have completed the demonstration vehicle in the short time that we did without the support and collaboration of the consortium.”

SwRI built on experience from previous heavy-duty low-NOx projects to develop a novel aftertreatment system specifically adapted to the hydrogen exhaust environment. Paired with the H2-ICE’s already low engine emissions, the addition of the aftertreatment system reduces NOx emissions to 0.008 g/hp-hr with aged catalysts, which is well below the 2027 EPA limit of 0.035 g/hp-hr.

Challenges met

SAE Media interviewed Williams at SAE COMVEC 2024 where he discussed the driving forces behind the development of the truck as well as some of the obstacles that the consortium overcame to meet their goals.

“Obviously we’ve got a big challenge of decarbonizing transportation,” he said. “We’re seeing the challenges and the limitations of the current zero-carbon offerings, as well as just the practicality of being able to make enough of those vehicles at scale to replace everything that’s on the road right now. That was a big part of the motivation behind this project.”

“This truck was created to show what technology options are available that we think will help us to ramp things up a lot faster and eventually will be more cost effective,” he explained. “We’ve got all this great technology. We’ve been evolving technology for combustion engines over the last hundred years. So why not take all that technology and use it?”

Williams also discussed the emissions side of SwRI’s efforts. “Another big part of this program is to demonstrate the emissions because that’s the other half of the question everybody asks. So, we’ve been able to demonstrate through this vehicle that we’ve significantly lowered NOx more than any other program or engine that’s out there.

“For context, the current standards that have been settled on for 2027 are 35 milligrams per horsepower hour. We set out with the goal of at least meeting that,” he continued. “But then California also has a voluntary ultra-low NOx standard of 20 milligrams. What we were able to demonstrate is below 10 milligrams on every regulatory cycle.”

SwRI built upon knowledge gained in past low-NOx demonstrations. “We started with the same architecture of the aftertreatment system, but thanks to the benefits of hydrogen, we were able to get the NOx emissions about ten times lower than a typical diesel engine,” Williams said. “That allowed us to significantly simplify the aftertreatment system. We’ve taken out most of the precious metal content and about half of the volume of catalyst. The DEF consumption is about a tenth of what you would expect from a diesel engine.”

Controlled combustion

Williams also discussed the challenges of achieving clean combustion with hydrogen fuel. “Hydrogen has a very low lean flammability limit,” he explained. “It’s a little more analogous to natural gas. You can run about one and a half Lambda [with CNG], that's one and a half times the air-fuel ratio before you start to get into unstable combustion.”

“One of the big challenges for on-road hydrogen combustion is transient response,” Williams noted. “Hydrogen is great for stationary generation, but when you start putting it on-road application, things get more difficult. With hydrogen, we can go well beyond that to about two and a half Lambda. There’s a sharp curve for engine-out NOx and around two [Lambda] is where that curve gets steep. If we can stay above two Lambda, then we can keep the engine-out NOx very low. We just have to make sure that our Lambda stays within that window.”

Williams continued, “If you go too rich during conditions such as throttle tip in, then two things happen. First, you get a NOx spike because you’re on that steep part of the curve on the NOx trade-off already, and then your temperature starts to spike at the same time. That’s where supercharging the engine has been really beneficial because at tip in we can get Lambda on demand.”

SuperTurbo provided one of its combination supercharger/turbocharger units for the demonstration truck. “What the SuperTurbo unit does is decouple the turbine from the compressor, which enables us to control the compressor relative to the turbine,” Williams explained. “We can actually pull power off of the crank via the pulley on the front of the engine. So that power can be delivered straight to the compressor to supercharge the engine.”

“Then on the back end, when we’ve got excess energy going through the turbine, we can actually pull that back through the transmission and put it back into the engine to turbo compound,” he continued. “One of the concerns about supercharging has always been that it’s a parasitic loss on the engine. But with the turbo compounding over the full cycle of the engine, it kind of balances itself out because you’re pulling out just as much energy as we’re putting in.”

Williams also discussed the injection system that the consortium used for the demonstration truck. The low-pressure port injection system operates between 10 and 15 bar (145 and 218 psi). “The reason for choosing port injection was component availability and overall complexity,” he said. “To do direct injection right, you have to design the cylinder head around it.”

Williams continued, “Hydrogen is a little counterintuitive because it’s so diffusive. You would think that you inject it and it’s just going to mix perfectly and be great. Well, it doesn’t really work like that because when you inject it into that air in the cylinder, the air just acts like a brick wall. So, most of these engines are designed with a lot of swirl for that’s beneficial for diesel combustion.

“If you have a flat cylinder head and a tangential port, that makes the air swirl around the outside. But, when you go to direct injection, you inject the fuel right in the middle and all of your air just swirls around it and it never mixes. So. if you wanted to go that route, you’d have to redesign things and rethink things for DI.”

The aggressive timeline for the project was a major limitation. “We wanted to take available hardware and we had a very short timeframe, about 18 months from start to finish,” Williams said. “So we didn’t have time to go fully redesign the cylinder head to consider DI.”

Sporadic spark

As is the case with any prototype vehicle, there were a few stumbling blocks along the way to making the SwRI demonstrator work as intended. One of those challenges was on the ignition side. “We started with the natural gas version of the X15 engine because it already had a spark plug, which is beneficial for hydrogen. But the rest of the ignition system is fairly unique for this engine,” Williams said.

“Usually, when you’re burning a hydrocarbon fuel, you end up with a lot of charged ions that are the byproducts of combustion and they sit there in the gap of the plug. So even after your spark is extinguished, you have these ions in there and you’ll still have some flow of current through the gap. But with hydrogen combustion, we don’t get that,” he explained. “It’s a very abrupt end. So rather than a gradual bleed down of the charge on the coil, we have some residual charge that stayed there. And then when we come around to the next combustion cycle, the pressure drops and we’re adding new fuel and the dielectric properties of the hydrogen.

“Once those ions get into your spark gap, they’re ideal for initiating another spark because we’ve got that residual charge on the coil,” Williams continued. “So we would get these random spark events during intake when the valve is open. It showed up as a misfire and it was really baffling to us because the engine would just stumble and be misfiring. But the reason that it’s misfiring is because you’ve consumed all the fuel that was meant to go into the cylinder while it was still in the intake manifold.”

The solution to the issue came from previous research that SwRI had conducted 20 years ago. “We looked at just about everything we could on the engine itself,” he said. “And we ended up going back and finding a paper that was written by SwRI around 2003 that talked about this exact phenomenon.

“What was happening is called ‘ghost spark,’ which is when you get random spark plug discharges that you didn’t plan for,” he explained. “To solve the issue, we tapped one of our consortium members (SEM) who provided a capacitive discharge ignition system. That system charges and full discharges on every cycle, which therefore eliminated our issue with residual charging when using traditional inductive coils.”

The future is now

More than anytime in the last century, investment in alternative propulsion systems is producing results that can be put into use today. While there will always be growing pains, the collective that came together to create SwRI’s H2 demonstrator have shown that the knowledge and tools exist to make this technology both commercially and economically viable.

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